The present invention relates to a solid state relay capable of high speed switching operation.
For solid state switching application, metal-oxide-semiconductor (MOS) field-effect transistors are employed as switching elements to take advantage of their high speed switching capability. One typical example of such solid state switching devices or relays is shown and described in U.S. Pat. No. 4,227,098 issued to Brown et al. This relay comprises a light-emitting diode connected between a pair of input terminals, an array of photodiodes series connected between the gate electrode and substrate electrode of a MOS field-effect transistor. The photodiode array is optically coupled to the light-emitting diode to generate a voltage in response to radiation therefrom when a forward current is supplied to the input terminals. The source and drain (current carrying electrode) of the transistor are connected respectively to a pair of output terminals. In the absence of the voltage, the impedance between the output terminals is high representing an open circuit and in the presence of the voltage the impedance is low representing a contact closure. A resistor is connected across the gate and substrate electrodes (voltage receiving electrodes) of the transistor to discharge energy stored therein to allow the transistor to turn off rapidly in response to the turn-off the light-emitting diode. Since the discharge resistor is also in parallel connection with the photodiode array, however, there is a loss of voltage which is applied to the transistor, resulting in an increase in the turn-on time.
Another solid state relay is disclosed in U.S. Pat. No. 4,390,790 issued to Rodriguez. This relay comprises an enhancement mode MOS field-effect transistor as a switching element and a depletion mode MOS field-effect transistor having current carrying electrodes connected to the voltage receiving electrodes of the switching transistor to provide a discharge path. This solid state relay additionally includes a second array of photodiodes optically coupled to the light-emitting diode to provide a photogenerated voltage to the gate electrode of the depletion mode MOS field-effect transistor to turn it off in response to the turn-on of the light-emitting diode. The voltage generated by the first photodiode array in response to the turn-on of light-emitting diode causes the enhancement mode switching transistor to turn on. Thus, the voltage supplied from the first photodiode array to the switching transistor in response to the turn-on of the light-emitting diode encounters no loss. A discharge resistor is connected across the second photodiode array to enable discharge of the depletion mode transistor upon turn-off of the light-emitting diode to allow it turn on to provide a discharge path for the energy stored in the enhancement mode switching transistor. Since the impedance of the discharge path is much smaller than the discharge resistor of U.S. Pat. No. 4,227,098, a faster turn-off operation can be achieved.
However, the solid state relay of U.S. Pat. No. 4,390,790 is still not satisfactory in terms of both turn-on and turn-off times. More specifically, the normally turn-on condition of the depletion mode discharge transistor tends to adversely affect on the second photodiode array so that its photogenerated voltage is prevented from rising sharply in response to the reception of radiation from the light-emitting diode. In addition, the discharge resistor connected across the second photodiode array presents a bypass current flow passage which tends to counteract the storage of energy on the gate electrode of the depletion mode transistor. As a result, the turn-on time of the depletion mode transistor is about 600 microseconds, a value which is too large for high speed switching applications. Likewise, during the turn-off of the relay, the energy stored in the depletion mode transistor must be discharged through the resistor before the enhancement mode switching transistor is turned off. A typical turn-off time of the prior art solid state relay is also 600 microseconds.